The postal system and high volume package shipping industry use tilt tray conveyor systems to sort bundles of letters and packages according to their respective destinations. Specialized sorters sort a bundle or package by destination zip code. During operation, an input stream of parcels is placed on a tilt tray conveyor and sorted into multiple output streams. The conveyor sorts the packages by tilting and ejecting them to either another appropriate conveyor or to an intermediate destination such as an unloading station.
Prior art tilt tray conveyor systems comprise a series of tilt tray carts linked together in a continuous loop. According to one known tilt tray conveyor system, the trays are secured to an endless drive chain, which pulls the trays around the loop. See Muller U.S. Pat. No. 3,662,874, issued May 16, 1972. According to another known tilt tray conveyor system known as the Mantissa Scorpion, linear induction motors (LIM's) are disposed at intervals around the loop for acting on a horizontally or vertically disposed plate (drive element) on each cart. The frame of each cart is T-shaped with a single axle, so that each cart depends on an adjoining cart for support.
FIG. 1 illustrates a conventional loop 5 of LM-driven tilt tray carts 10 connected head to tail and mounted on an endless, generally oval-shaped rail 12. The continuous loop of carts creates significant inefficiencies in the conveyor system. First, the system's strength depends literally on its weakest link. For example, if one cart 10 or its tray fails, the entire system must be stopped until the cart is repaired or replaced. Second, inefficient loading frequently occurs. The system may skip carts to maintain conveyor speed. This creates a situation in which empty carts are pulled around the loop, thereby resulting in wasted energy and system capacity. Additionally, some applications require large distances between input and output streams. Increased costs associated with longer cart chains may prohibit using a continuous chain conveyor system in a large loop.
Referring now to FIG. 2, transferring parcels between multiple loops 5A and 5B requires unloading the parcel from loop 5A and transferring it to the other loop 5B by a gravity slide 22 which feeds parcels to conventional conveyor belt 24. Belt 24 delivers the parcels to a powered induction station 26 which loads it onto a tray of a cart 10 in loop 5B. The potential for parcel damage occurs with each transfer to and from the carts 10. This manner of transfer between loop 5A and loop 5B introduces many opportunities for the item to be damaged because moving an item to or from trays involves subjecting the item to forceful impacts.
Inefficiencies caused by the method of locomotion also exist. According to another known conveyor design called the NovaSort, a product of Siemens ElectroCom, L.P., a train or segment of tilt tray carts connected end to end is drawn by a leading cart having an engine in the manner of a monorail. The lead cart draws power from a sliding electrical contact on the rail. This design suffers the customary drawbacks of systems that rely on sliding electrical contacts. In addition, the carts of each segment contain a solenoid that actuates the tilting mechanism on each cart, thus adding to the weight and complexity of the system.
Accordingly, a low-maintenance cart system is needed that reduces the potential for parcel damage created by cart transfers between loops.